Structural effects in three-ring mesogenic derivatives of p-carborane and their hydrocarbon analogues

Abstract

Several series of structurally related three-ring esters containing benzene, cyclohexane, bicyclo[2.2.2]octane and p-carborane rings were synthesized and their mesogenic properties investigated using thermal analysis and optical microscopy. Carborane derivatives show only nematic phases, while the richest smectic polymorphism (up to three phases) was observed in the biphenyl series D. The structure–property relationships were studied by comparison of T _NI between series. The ring effectiveness in stabilization of nematic phases generally follows the order carborane<benzene∼cyclohexane<bicyclo[2.2.2]octane. The results indicate that fill fraction plays a significant role in the stabilization of the mesophase. A notable positional effect of the carborane ring on T _NI was also observed.

Notes

^†The estimated free energy of the diaxial conformer of 1C is about 1.5 kcal mol⁻¹ higher than that for its isomer 2D. Based on tabularized A values for monosubstituted cyclohexanes [20].

^‡The difference in ΔG^‡ for rotation around the ring–(COOH) and ring–Ph bonds has negative correlation with the ΔT _NI for isomeric pairs (slope −0.30, r=0.93). The energies were obtained at the MP2/6-31G* level of theory with B3LYP/6-31G* thermodynamic corrections.